This invention relates to the testing of optical fibers, and, more particularly, to the apparatus used to mechanically test the payout characteristics of optical fibers from bobbins.
Optical fibers are strands of glass fiber processed so that light beams transmitted therethrough are subject to total internal reflection. A large fraction of the incident intensity of light directed into the fiber is received at the other end of the fiber, even though the fiber may be hundreds of meters long. Optical fibers have shown great promise in communications applications, because a high density of information may be carried along the fiber and because the quality of the signal is less subject to external interferences of various types than are electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight and made from a highly plentiful substance, silicon dioxide.
Glass fibers are typically fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, and processing the preform to a fiber. The optical fiber is coated with a polymer layer termed a buffer to protect the glass from scratching or other damage. As an example of the dimensions, in a typical configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the fiber plus the polymer buffer is about 250 micrometers (approximately 0.010 inches).
For such very fine fibers, the handling of the optical fiber to avoid damage that might reduce its light transmission properties becomes an important consideration. The fibers are typically wound onto a cylindrical or tapered bobbin with many turns adjacent to each other in a side by side fashion. After one layer is complete, another layer of fiber is laid on top of the first layer, and so on. The final assembly of the bobbin and the wound layers of fiber is termed a canister, and the mass of wound fiber is termed the fiber pack. When the optical fiber is later to be used, the fiber is payed out from the canister in a direction parallel to the axis of the bobbin.
It has been found by experience that, where the fiber is to be payed out from the canister in a rapid fashion, as for example over a hundred meters per second, the turns of optical fiber must be held in place on the canister with an adhesive. The adhesive holds each turn of fiber in place as adjacent turns and layers are initially wound onto the canister, and also as adjacent turns and layers are payed out. Without the use of an adhesive, payout of the fibers may not be uniform and regular, leading to snarls or snags of the fibers that damage them or cause them to break as they are payed out.
It is important to test the procedures for winding the optical fiber onto the bobbins, the effectiveness of the adhesive, and any environmental degradation, under realistic payout conditions. A standard test to evaluate an optical fiber canister is to draw the optical fiber from the bobbin, parallel to its longitudinal axis using a payout drive. Payout occurs at a very high linear rate that is typically over one hundred meters per second.
To prevent excessive transverse vibration of the optical fiber and ensure that its movement uniformly converges on the location where it is pulled at this high rate, there is typically provided a conical guide between the canister and the payout wheel that pulls the optical fiber from the bobbin. This guide dampens any excessive transverse vibrations of the optical fiber during the test. Without such a guide, it is virtually impossible to pay out any substantial length of optical fiber without the fiber becoming misaligned. If misalignment occurs, the optical fiber fails.
While the guide works well for relatively short lengths of optical fiber and the testing is successful, it has not been possible to test longer lengths of optical fiber using this approach. It has been found that, when the length of the optical fiber paid out exceeds about 5 kilometers, the optical fiber almost invariably breaks, even when the conical guide is used.
Consequently, it has not been possible to conduct payout tests of optical fibers from canisters in long test lengths, typically greater than about 5 kilometers. For same current applications, much longer payout test lengths, at least about 20 kilometers, are required to validate winding procedures, adhesive type and application; and any adverse effects of extended storage on the payout characteristics. Accordingly, there exists a need for a better testing approach for evaluating the payout characteristics of optical fibers from canisters. The present invention fulfills this need, and further provides related advantages.